A therapy that improves your health and helps weight loss while you just sit there? Anastasia Stephens took a deep breath to investigate

My head is spinning, my limbs are tingling and I'm beginning to feel strangely high. No, these are not the symptoms of a narcotic influence, but the side-effects of breathing the sort of atmosphere you might inhale at an altitude of 5,000m. Given the air is so thin – just 11 per cent oxygen, compared to the normal 20 per cent – my body has assumed that I am perched atop one of Europe's highest mountains. In fact, I'm comfortably seated at sea-level indulging in the latest fitness program available at The Altitude Centre in London. Mountain Air Therapy or Intermittent Hypoxic Training (IHT) sounds like an unlikely way of increasing vitality or fitness. But short periods of oxygen deprivation can actually increase fitness levels and endurance. Professional athletes have long known about the benefits of training in the thin air of the Andes or the Alps. Intermittent Hypoxic Training (IHT) is the logical conclusion of this practice – mountain air, without the need for a mountain.

"Altitude training makes your body work better in many ways," explains Richard Pullen, founder of The Altitude Centre, who used the technique while preparing for the Atlantic Rowing Challenge last year. "As there is less oxygen at altitude, the body adapts to become more efficient. Your heart muscle beats harder and so becomes stronger. You breathe more deeply, increasing lung capacity. New capillaries grow to get the limited supply of oxygen to tissues. All these changes mean that when you return to sea-level, the oxygen levels you are normally adapted to breathe take you further."

Cav Burke, 49, a floor polisher from Epping, used IHT to help train for his ascent of Mount Kilimanjaro. "Climbing Kilimanjaro was my dream and a great reason to lose weight and get fit," says Burke. "When I started, I was very overweight at 17 stone, and my blood pressure was high. Running and regular sessions at the gym helped me lose around six stone. After that, despite my training, I just couldn't shift any more weight, and my blood pressure was still above normal." Burke tried IHT in October to help his body acclimatize to high altitudes, and to see if it could help him lose extra weight. "I was pretty surprised – the course of IHT left me seven pounds lighter. I found my recovery rate after a workout at the gym was getting faster. I was even happier to discover that after three weeks of IHT, my blood pressure had fallen by 15 per cent, from high to normal."

Several studies attest to the benefits of such treatment. In the treatment of asthma, IHT has enabled patients to reduce their medication, it has also been shown to improve energy levels in patients suffering from chronic fatigue. Yet, unlike climbing a mountain, where your body adapts to gradually lower levels of oxygen, IHT deprives the body of oxygen in short, sharp bursts. During treatment, oxygen saturation levels in the blood, heart rate and blood pressure are closely monitored. This enables oxygen levels to be reduced safely. One immediate effect of IHT is that levels of the feel-good chemicals dopamine and serotonin increase noticeably – after a few minutes of breathing 11 per cent oxygen, I felt euphoric but calm. And, as anyone who has ever walked at altitude knows, thin air acts as a diuretic, ridding the body of excess water. One hour-long session had me rushing to the loo three times.

While short-term benefits, such as a flatter stomach and a sense of euphoria, are an immediate draw, it's the more dramatic effects of undergoing the treatment over time that are bringing new converts to the therapy. Pullen suggests that, after a course of 15 days, levels of red-blood cells – cells that carry oxygen to tissues around the body – can increase by up to a third. New capillaries grow to supply more oxygen to tissues. Heart rate and blood pressure drop, and, as the tissues adapt to take up more oxygen, the cells in your body become more active. Your metabolism changes, too – and areas of persistent fat disappear. "From a fitness perspective, it's brilliant," says Pullan. "Studies have confirmed that athletes can improve their anaerobic performance by 40 per cent and aerobic performance increases by 20 per cent." Breathing 11 per cent oxygen means I am forced to inhale slowly and deeply, and my heart beats faster to get the limited oxygen round my body. The overall effect means I get a workout without having to move – as a workout for the immobile, it's hard to beat. I only had three sessions, but after each hour I felt invigorated and I was breathing more deeply. My heart rate went down from 60bpm to 57bpm and my blood pressure dropped from 117/79 to 100/65. Not bad for three hours of doing nothing but breathe. The benefits of IHT can last for up to three months and are maintained with five-day top-up courses. The Altitude Centre, Unit 115, 78 Marylebone High Street, London W1 (www.altitudecentre.com); call 0870 950 4479. Editor’s note: For information about IHT in the United States, please visit: http://www.hypoxictent.com/.